﻿GEOLOGY OF THOUSAND ISLANDS REGION 25 



the similarly situated inclusions of the Grenville rocks them- 

 selves. The same conditions prevail in general over the much 

 more extensive Precambric areas of eastern Canada. Recently, 

 however, Miller and Knight have announced the discovery, in 

 central Ontario, of a basement to the Grenville formation, Gren- 

 ville limestone being found resting on an ancient lava flow, whose 

 surface is thought to show signs of slight previous wear. 1 Miller 

 and Knight correlate this old lava, or greenstone, with the oldest 

 known formation of the upper lake region, the Keewatin, which 

 consists mainly of greenstones, old lava flows and beds of frag- 

 mental volcanic materials. There are present, however, some 

 associated sediments, and Miller and Knight regard the Grenville 

 as of Keewatin age. These are most important results and if 

 future work fully establishes these correlations, it will follow 

 that the Keewatin has steadily increasing sedimentary content 

 and less and less volcanic material as it is followed eastward. 

 By the time New York is reached the greenstones have entirely 

 disappeared, so far as is known. At least no rocks similar to 

 them have ever been discovered in the New York Precambric. 

 It should also be stated that Adams is not disposed to accept the 

 reference of the Grenville to the Keewatin on the basis of the 

 evidence yet in hand, believing a reference to the next overlying 

 group, the Huronian, to be more probable. 2 



However this may be, the difficulty of accounting for the dis- 

 appearance of the old floor of deposit is not helped, but merely 

 pushed a stage further back. Miller and Knight speak of only 

 slight erosion of the old lava flow prior to the deposit of the 

 Grenville limestone upon it. It is of course possible that this 

 may be merely an interbedded flow of Grenville age and itself 

 rest upon other Grenville sediments. But in any case these 

 Keewatin lava flows and fragmental deposits are surface deposits 

 and require the presence of a floor on which they were laid down 

 just as much as do the Grenville sediments; but no such floor 

 to the Keewatin is known. It is always found resting on Lauren- 

 tian granite gneisses of igneous origin, or upon yet younger 

 igneous rocks which invaded it from beneath in molten condition, 

 cut it to pieces, and apparently engulfed and assimilated its basal 

 portion along with the floor upon which it rested. Precisely 

 these same conditions prevail in general in respect to the Gren- 

 ville and its former floor. 



1 Bureau of Mines, Ontario. 16th An. Rep't, pt 1, p. 22-23. 



2 Adams, F. D. Jour. Geol. 16:634-35. 



